The present invention relates to semiconductor devices and manufacturing process, and more particularly to methods for forming retrograde channel profiles within a semiconductor devices.
The principal elements of a typical metal-oxide-semiconductor (MOS) device generally comprise a semiconductor substrate on which a gate electrode is disposed. The gate electrode is typically a heavily doped polysilicon semiconductor. Heavily doped source/drain regions are formed in the semiconductor substrate and are connected to source/drain terminals. A channel region is formed in the semiconductor substrate beneath the gate electrode and separates the source/drain regions. The gate electrode is generally separated from the semiconductor substrate by an insulating layer to prevent current from flowing between the gate electrode and the source/drain regions or channel regions.
In operation, an output voltage is typically developed between the source and drain terminals. When an input voltage is applied to the gate electrode, a transverse electrical field is set up in the channel region. By varying the transverse electric field between the source and drain regions, it is possible to modulate the conductance of the channel region between the source and drain regions. In this manner, an electric field controls the current flow through the channel region. The channel is typically lightly doped with an impurity type opposite to that of the source/drain regions, and the impurity concentration profile is typically uniform from the surface toward the direction of depth, as shown by line a in FIG. 3.
In particular MOSFET devices, a channel implantation process is frequently performed to augment the substrate doping of the same conductivity. Thus, for an NMOS device, a p-type impurity is ion implanted into the substrate and channel region, and for a PMOS device, an n-type impurity is ion implanted into the substrate and channel region. The purposes of this channel implantation are typically to adjust threshold voltages and improve short channel characteristics, and many approaches have been introduced, such as a retrograde well structure for that purposes. As shown by curve b in FIG. 3, the retrograde well structure generally has an impurity concentration peak deep under the surface. A method for manufacturing a retrograde well structure in semiconductor devices is disclosed in U.S. Pat. No. 5,726,488 by Watanabe, et al. According to Watanabe, et al., a retrograde well structure is achieved by forming an epitaxial layer on a semiconductor substrate having a highly doped boron layer, formed by ion implantation, in its surface. As shown in FIG. 1A, a lightly doped P type silicon substrate 10 is prepared and the first oxide film 12 is formed thereon. A first photoresist layer 14 is formed on the first oxide film 12. In FIG. 1B, after patterning the first photoresist layer 14 to expose a portion of the top surface of the first oxide film 12, boron is ion implanted, as shown by arrows A, into the first oxide film 12 at an implantation dosage of between 5xc3x971012 atoms cmxe2x88x922 to 2xc3x971013 atoms cmxe2x88x922 and at an implantation energy of 50 KeV, thereby forming a high concentration layer 16 in the surface of the substrate 10. As shown in FIG. 1C, after removing the first photoresist layer 14 and the first oxide film 12, an epitaxial layer 18 is formed on the substrate 10 including the high concentration layer 16 at a thickness of between 0.8 xcexcm to 1.7 xcexcm. Then, as shown in FIG. 1D, a second oxide film 20 is formed on the epitaxial layer 18, and a nitride film 22 is formed on the second oxide film 22. A second photoresist layer 24 is formed on the nitride film 22. As shown in FIG. 1E, after patterning the second photoresist layer 24 and the nitride film 22 to expose a portion of the surface of the epitaxial layer 18 overlying the high concentration layer 16, BF2 ions are implanted, as shown by arrows B, into the second oxide film 20 at an implantation dosage of 2xc3x971012 atoms cmxe2x88x922 and an implantation energy of 60 KeV, thereby forming a low impurity region 26 within the epitaxial layer 18. As shown in FIG. 1F, after removing the second oxide film 20, the nitride film 22, and the second photoresist layer 24 from the top surface of the epitaxial layer 18, an annealing process is performed to repair the lattice damage by the previous ion implantation. A gate oxide layer 28 is then formed on the low impurity region 26, and local isolation regions 29 are formed on the epitaxial layer 18. In FIG. 1G, a gate electrode 30 is formed on the gate oxide 28 with sidewall spacers 32. Then, n-type active regions 34 are formed by ion implanting arsenic ions at an implantation dosage of 3xc3x971016 cmxe2x88x922 and at an implantation energy of 80 KeV.
Although the method disclosed in Watanabe et al. provides a relatively simple method to form a retrograde channel profile, the implanted boron atoms contained in the high concentration layer 16 are easily diffused into the overlying epitaxial layer 18 by the subsequent fabrication steps e.g., annealing, thereby degrading the carrier mobility of the channel region and the channel inversion charge density.
Thus, there is a continuing need for improved methods that form a super-steep retrograde impurity profile to improve the channel carrier mobility and the channel inversion charge density, thereby improving devices"" speed performance.
An advantage of the present invention is an efficient and production-worthy method of manufacturing a semiconductor device having a retrograde channel profile to improve devices"" speed performance.
Another advantage of the present invention is a simplified and improved method of manufacturing a super steep retrograde channel profile.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method of manufacturing a semiconductor device, which method comprises: forming a high impurity concentration layer of a first conductive type on a main surface of a semiconductor substrate; introducing a non-dopant into the high concentration impurity layer to form a diffusion cap layer near a top surface of the high concentration impurity layer; and forming a low impurity concentration layer of the first conductive type on the surface of the first semiconductor layer. The diffusion cap layer formed between the high impurity concentration layer and the low impurity concentration layer substantially prevents impurity atoms contained in the high concentration layer from diffusing into the low impurity concentration layer.
Another aspect of the present invention is a semiconductor device comprising: a silicon substrate having a main surface; a high impurity concentration layer of a first conductive type formed on the main surface of the silicon substrate, the high impurity concentration layer comprising a diffusion cap layer formed near a top surface of the high impurity concentration layer; and a low impurity concentration layer of the first conductive type formed on the top surface of the high impurity concentration layer.
Additional advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.